Ischemic heart disease typically results from an imbalance between the myocardial blood flow and the metabolic demand of the myocardium. Progressive atherosclerosis with increasing occlusion of coronary arteries leads to a reduction in coronary blood flow. Blood flow can be further decreased by additional events such as changes in circulation that lead to hypoperfusion, vasospasm or thrombosis.
Myocardial infarction accounts for approximately 20% of all deaths. It is a major cause of sudden death in adults.
Myocardial Infarction (MI) is one form of heart disease that often results from the sudden lack of supply of oxygen and other nutrients. The lack of blood supply is a result of closure of the coronary artery that nourishes the particular part of the heart muscle. The cause of this event is generally caused by arteriosclerosis “hardening of the arteries” in coronary vessels.
Formerly, it was believed that an MI was caused from a slow procession of closure from, for example, 95% then to 100%. However, an MI can also be a result of minor blockages where, for example, there is a rupture of the cholesterol plaque resulting in blood clotting within the artery. Thus, the flow of blood is blocked and downstream cellular damage occurs. This damage can cause irregular rhythms that can be fatal even though the remaining muscle is strong enough to pump a sufficient amount of blood. As a result of this insult to the heart tissue, scar tissue tends to naturally form.
Even though systemic drugs presently exist to treat MI, such as ACE-inhibitors and Beta-blockers, a significant portion of the population that experiences a major MI ultimately develops heart failure. An important component in the progression to heart failure is remodeling of the heart due to mechanical forces resulting in uneven stress and strain distribution in the left ventricle. Once an MI occurs, remodeling of the heart begins. The principle components of the remodeling event include myocyte death, edema and inflammation, followed by fibroblast infiltration and collagen deposition, and finally scar formation. The principle component of the scar is collagen. Since mature myocytes of an adult are not regenerated, the infarct region experiences significant thinning. Myocyte loss is the major etiologic factor of wall thinning and chamber dilation that may ultimately lead to progression of cardiac myopathy. Myocyte death can and does occur. In other areas, remote regions experience hypertrophy (thickening) resulting in an overall enlargement of the left ventricle. This is the end result of the remodeling cascade. These changes in the heart result in changes in the patient's lifestyle and their ability to walk and to exercise. These changes also correlate with physiological changes that result in increase in blood pressure and worsening systolic and diastolic performance.
FIGS. 1A-1C illustrates blood flow by longitudinal cross sectioning of the artery. FIG. 1A illustrates a normal unobstructed artery. FIG. 1B illustrates artery damage due to a tear or spasm. FIG. 1C illustrates an artery with plaque build-up that reduces the blood flow demonstrated by the blocked blood cell above the atherosclerotic mass. Fat and cholesterol build up at the site of damage. This mass can be detected by methods currently available which methods include an echocardiogram, single photon emission computed tomography (SPECT), magnetic resonance imaging (MRI), or an angiogram.
FIGS. 2A-2B illustrate the progression of heart damage once the build-up of plaque induces an infarct to occur. The most common pathogenesis of this disease is occlusive intra-coronary thrombus where a thrombus is covering an ulcerated stenotic plaque. This causes approximately 90% of transmural acute myocardial infarctions. Other possible triggers of an MI are vasospasms with or without coronary atherosclerosis and possible association with platelet aggregation. Another possible trigger is embolisms from left-sided mural thrombosis, vegetative endocarditis or a paradoxic embolism from the right side of the heart through a patent foramen ovale. FIG. 2A illustrates a site where blockage and restricted blood flow can occur from any of the indicated causes. FIG. 2B illustrates the extensive damage to the left ventricle that can be a result of the lack of oxygen and nutrient flow carried by the blood to the inferior region left ventricle of the heart. This area will likely undergo remodeling and eventually a scar will form and a non-functional (an area that does not contract) area will exist.
Significant atherosclerotic build-up can reduce the arterial lumen and reduce blood flow. Build-up is capable of rupturing resulting in a total or partial occlusion of the artery. Complete coronary occlusion will lead to an acute MI. Thus the T-cells, platelets, fibrin and multiple other factors and cells are blocked from progression through the blood stream and the result is an inadequate vascular supply as seen. This leads to myocyte death. Myocyte death, in addition to fibrosis in the form of collagen deposition, can lead to a compromised left ventricle and overload on the remaining myocytes. This process is further complicated by compensation of the remaining myocytes that hypertrophy (enlarge). This can cause the left ventricle to enlarge and if the cycle continues can result in eventual heart failure.
The morphological appearance of the infarcted heart tissue post-MI can vary. A transmural infarct involves the entire thickness of the left ventricular wall from the endocardium to the epicardium. It may extend into the anterior free wall and the posterior free wall. This damage may include extensions into the right ventricular wall. A sub-endocardial infarct may have multiple focal regions and a necrosis area may be confined to the inner one-third to one-half of the left ventricular wall. The evolutionary changes in a sub-endocardial infarct do not evolve the same as in a transmural MI.
Over time post-MI morphological changes occur. The gross morphological changes that occur over approximately a 7-week period are pallor of the myocardium that leads to some hyperemia, then a yellowing central to the damaged region. At approximately 15 days, the area is mostly yellow with soft vascular margins. This area eventually turns white from fibrosis. On a microscopic level, the initial examination reveals wavy myocardial fibers. Coagulation and necrosis with loss of cross striations occur followed by contraction bands, edema, hemorrhage, and neutrophilic infiltrate. Within 24-72 hours there is total loss of nuclei and striations and heavy neutrophilic infiltrate. Then macrophage and mononuclear infiltration begin resulting in a fibrovascular response. Once this fibrovascular response occurs then prominent granulation of the tissue follows. This ultimately leads to fibrosis and a scar is formed by about 7 weeks post MI.
FIGS. 3A-3B illustrate the occlusion of an artery that may lead to an MI. FIG. 3A illustrates the cross-section of a normal coronary artery with unobstructed lumen 301. The normal arterial wall 302 is made up of an intima layer 303, a media layer 304, and an adventitia layer 305. Within the arterial lumen, the intima is in direct contact with the flow of blood. This region is mostly made up of endothelial cells. The media layer is mostly smooth muscle cells and extracellular matrix proteins. Finally, the adventitia layer is primarily made up of collagen, nerves, blood vessels and lymph vessels. FIG. 3B illustrates a coronary artery with atherosclerosis. In this example, this artery is about 50 percent occluded (only 50 percent of the arterial lumen is free of obstruction). Thus, the obstructed artery may lead to damage observed in a ventricle of an MI subject.
After an MI has occurred, three layers of tissue can be distinguished. The infarct region has (1) the region of significant necrosis/apoptosis tissue (2) the border zone that consists of a large concentration of apoptotic and necrotic tissue as well as viable tissue and (3) the unaffected region that consists of mainly viable tissue. In the border zone the cells exist in an oxygen-deprived state due to the damage from the MI.
FIGS. 3C-3J illustrate the details of a post-MI remodeling of the ventricle. The progression of heart failure after an MI is a result of the remodeling of the heart after the infarct. The remodeling process causes the infarcted region of the heart to stretch and become thinner causing the left ventricular diameter to increase. As the heart continues to remodel, the stresses on the heart increase. FIG. 3C illustrates the cross striations 306 and central nuclei 307 of a healthy myocyte population.
FIGS. 3D-3J depict the progression of the remodeling of the ventricle post-MI. FIG. 3D illustrates an early acute MI. Here, there are prominent pink contraction bands that are indicated by reference number 308. FIG. 3E illustrates the increasing loss of striations and some contraction bands. The nuclei in this illustration are incurring karyolysis (a stage of cell death that involves fragmentation of a cell nucleus; the nucleus breaks down into small dark beads of damaged chromatin) 309. In addition, the neutrophils are infiltrating the damaged myocardial region. FIG. 3F illustrates an acute MI. The loss of nuclei and loss of cross striations are evident. There is extensive hemorrhaging on the infarct border 310. FIG. 3G illustrate the prominent necrosis and hemorrhaging 310, as well as the neutrophilic infiltrate 311. Subsequently, a yellowish center is formed within the damaged area with necrosis and inflammation surrounded by the hyperemic border. After 3-5 days post-MI, the necrosis and inflammation are extensive. There is a possibility of rupture at this point. FIG. 3H illustrates approximately one week after the MI with capillaries, fibroblasts and macrophages filled with hemosiderin (hemosiderin is a long-term reserve of iron in tissues) 312. In two to three weeks, granulation is the most prominent feature observed. FIG. 3I illustrates extensive collagen deposition 313 seen after a couple of weeks. Collagenous scarring occurs in subendocardial locations in remote myocardial infarct regions. FIG. 3J illustrates myocytes 314 after several weeks of healing post-MI. They are hypertrophied with large dark nuclei 315 and interstitial fibrosis 316. Interstitial fibrosis is also observed. These enlarged cells contribute to the enlarged left ventricle.
A complication of an MI is an aneurysm that looks like a bulge in the left ventricular wall. The aneurysm is made up of non-functional tissue that is unable to contract. Therefore, the ejection and stroke volume of the heart are reduced. Additionally, parts of this mass can form a mural thrombus that can break off and embolize to the systemic circulation.